An electrophotographic light-sensitive material may have various structures depending upon the characteristics required or an electrophotographic process being employed.
An electrophotographic system in which the light-sensitive material comprises a support having thereon at least one photoconductive layer and, if necessary, an insulating layer on the surface thereof is widely employed. The electrophotographic light-sensitive material comprising a support and at least one photoconductive layer formed thereon is used for the image formation by an ordinary electrophotographic process including electrostatic charging, imagewise exposure, development, and, if necessary, transfer.
Furthermore, a process of using an electrophotographic light-sensitive material as an offset master plate for direct plate making is widely practiced.
A binder which is used for forming the photoconductive layer of an electrophotographic light-sensitive material is required to be excellent in the film-forming property by itself and the capability of dispersing therein a photoconductive powder as well as the photoconductive layer formed using the binder is required to have satisfactory adhesion to a base material or support. Also, the photoconductive layer formed by using the binder is required to have various charging capacity, less dark decay, large light decay, and less fatigue before light-exposure and also have an excellent photographing property that the photoconductive layer stably maintains these electrostatic properties to the change of humidity at photographing.
Binder resins which have conventionally used include silicone resins (e.g., JP-B-34-6670, the term "JP-B" as used herein means an "examined published Japanese patent publication"), styrene-butadiene resins (e.g., JP-B-35-1960), alkyd resins, maleic acid resins, polyamides (e.g., JP-B-35-11219), polyvinyl acetate resins (e.g., JP-B-41-2425), vinyl acetate copolymers (e.g., JP-B-41-2426), acrylic resins (JP-B-35-11216), acrylic acid ester copolymers (e.g., JP-B-35-11219, JP-B-36-8510, and JP-B-41-13946), etc.
However, in the electrophotographic light-sensitive materials using these binder resins, there are various problems such as 1) the affinity of the binder with photoconductive powders is poor thereby reducing the dispersibility of the coating composition containing them, 2) the charging property of the photoconductive layer containing the binder is low, 3) the quality (in particular, the dot image reproducibility and resolving power) of the imaged portions of copied images is poor, 4) the image quality is liable to be influenced by the surrounding conditions (e.g., high temperature and high humidity or low temperature and low humidity) at the formation of copies, and 5) the photoconductive layer is insufficient in film strength and adhesion, which causes, when the light-sensitive material is used for an offset master, peeling off of the photoconductive layer, etc. at offset printing to reduce the number of prints.
For improving the electrostatic characteristics of a photoconductive layer, various approaches have hitherto been taken. For example, incorporation of a compound having an aromatic ring or a furan ring containing a carboxy group or a nitro group either alone or in combination with a dicarboxylic anhydride in a photoconductive layer is disclosed in JP-B-42-6878 and JP-B-45-3073. However, the thus improved electrophotographic light-sensitive materials are yet insufficient in electrostatic characteristics and, in particular light-sensitive materials having excellent light decay characteristics have not yet been obtained. Thus, for compensating the insufficient sensitivity of these light-sensitive materials, an attempt to incorporate a large amount of a sensitizing dye in the photoconductive layer has been made. However, light-sensitive materials containing a large amount of a sensitizing dye undergo considerable deterioration of whiteness to reduce the quality as a recording medium, sometimes causing deterioration in dark decay characteristics, thereby satisfactory reproduced images are not obtained.
On the other hand, JP-A-60-10254 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a method of using a binder resin for a photoconductive layer by controlling the average molecular weight of the resin. That is, JP-A-60-10254 discloses a technique of improving the electrostatic characteristics (in particular, reproducibility at repeated use as a PPC light-sensitive material), humidity resistance, etc., of the photoconductive layer by using an acrylic resin having an acid value of from 4 to 50 and an average molecular weight of from 1.times.10.sup.3 to 1.times.10.sup.4 and the acrylic resin having an average molecular weight of from 1.times.10.sup.4 to 2.times.10.sup.5.
Furthermore, lithographic printing master plates using electrophotographic light-sensitive materials have been extensively investigated and, as binder resins for a photoconductive layer having both the eletrostatic characteristics as an electrophotographic light-sensitive material and the printing characteristics as a printing master plate, there are, for example, a combination of a resin having a molecular weight of from 1.8.times.10.sup.4 to 10.times.10.sup.4 and a glass transition point (Tg) of from 10.degree. to 80.degree. C. obtained by copolymerizing a (meth)acrylate monomer and other monomer in the presence of fumaric acid and a copolymer composed of a (meth)acrylate monomer and a copolymerizable monomer other than fumaric acid as disclosed in JP-B-50-31011, a terpolymer containing a (meth)acrylic acid ester unit with a substituent having a carboxylic acid group at least 7 atoms apart from the ester linkage as disclosed in JP-A-53-54027, a tetra- or pentapolymer containing an acrylic acid unit and a hydroxyethyl (meth)acrylate unit as disclosed in JP-A-54-20735 and JP-A-57-202544, and a terpolymer containing a (meth)acrylic ester unit with an alkyl group having from 6 to 12 carbon atoms as a substituent and a vinyl monomer containing a carboxyl group as disclosed in JP-A-58-68046.
However, none of these resins proposed have proved to be satisfactory for practical use in charging property, dark charge retention, electrostatic characteristics for photosensitivity, and the surface smoothness of the photoconductive layer.
Also, the practical evaluations on conventional binder resins which are said to be developed for electrophotographic lithographic master plates have found that they have problems in the aforesaid electrostatic characteristics, background staining of prints, etc.
For solving these problems, JP-A-63-217354 describes that the smoothness and the electrostatic characteristics of a photoconductive layer can be improved and images having no background staining are obtained by using a low-molecular weight resin (molecular weight of from 1,000 to 10,000) containing from 0.05 to 10% by weight a copolymer component having an acid group at the side chain of the copolymer as the binder resin, and also Japanese Patent Application 63-49817 and JP-A-63-220148 and JP-A-63-220149 described that the film strength of a photoconductive layer can be sufficiently increased to improve the printing impression without reducing the aforesaid characteristics by using the aforesaid low-molecular resin in a combination with a high-molecular resin (molecular weight of 10,000 or more).
However, it has been found that even in the case of using these resins, it is yet insufficient to keep the stable performance in the case of greatly changing the environmetal conditions from high-temperature and high-humidity to a low-temperature and low-humidity. In particular, in a scanning exposure system using a semiconductor laser light, the exposure time becomes longer and also there is a restriction on the exposure intensity as compared to a conventional overall simultaneous exposure system using a visible light, and hence a higher performance has been required for the electrostatic characteristics, in particular, the dark charge retention characteristics and photosensitivity.